1. Field of the Invention
The present invention relates to an image heating apparatus, adapted for use as a heat fixing apparatus for an image fixation of a recording medium bearing a toner image.
2. Related Background Art
In an image forming apparatus such as a printer or a copying apparatus, image formation is often executed by an electrophotographic process, and, in such process, a toner image is formed on a recording medium by a transfer method or a direct method, and is fixed by applying heat and pressure to such recording medium.
An image heating apparatus serving as a fixing apparatus for heat fixing the toner image has conventionally employed a heat roller system as shown in FIG. 6. This system is basically constituted of a heat roller 102 of a metallic material or the like provided therein with a heater 101, and an elastic pressure roller 103 pressed thereto, and a recording medium S bearing an unfixed toner image is introduced into a nip portion N (fixing nip) of the rollers 102, 103, and pinched and passed therein to heat the toner image t under heating and pressure.
However the fixing apparatus of such heat roller type requires a very long time for elevating the roller surface to a fixing temperature, because of a large heat capacity of the roller. For this reason, in order to achieve a prompt image outputting operation, the roller surface has to be controlled at a certain temperature even while the apparatus is not in use.
Therefore, Japanese Patent Application Laid-Open No. H04-44075 etc. propose an image heating apparatus of on-demand type of a configuration, in which a flexible sleeve (film) is employed in place for the highly rigid fixing roller and a heater is contacted with an internal surface of the sleeve, thereby forming a nip portion by the heater and the pressure roller, with the the flexible sleeve being therebetween.
Such image heating apparatus of on-demand type is generally constituted of a thin heat-resistant film (for example of polyimide), a heater (heat generating member) fixed at the side of a surface of the film, and a pressure roller provided at the side of the other surface of said film and opposed to the heater through the film for contacting a heated medium to the film.
When such apparatus is employed as a fixing apparatus, a recording medium is introduced into and passed by a nip portion (fixing nip) formed by a contact of the heater and the pressure roller through the film, whereby the recording medium is heated by the heater through the film to give the unfixed image with thermal energy, and whereby the toner image is fixed on the recording medium.
FIG. 7 is a schematic view showing a principal part of an image heating apparatus as explained above. A ceramic heater 113, constituting a heat generating member, is basically constituted of a thin oblong plate-shaped ceramic substrate having a longitudinal direction thereof perpendicular to the plane of the drawing, and a heat-generating resistor layer provided on a surface of the substrate, and is a heater of a low heat capacity showing a temperature increase over the entire surface with a steep start-up property by a current supply to the heat-generating resistor layer.
A holder 112 supports the heater 113. The holder 112 is a member formed by heat-resistant resin of a trough shape having a substantially semicircular cross section and having a longitudinal direction thereof perpendicular to the plane of the drawing. The heater 113 is fitted, with a heater surface thereof exposed downwards, in a groove formed on a lower face of the holder 112 and along the longitudinal direction thereof and fixed with a heat-resistant adhesive.
A cylindrical heat-resistant film 114 is loosely fitted around the holder 112 with the heater 113.
A pressurizing stay 111 is a rigid member having an inverted U-shaped cross section and a longitudinal direction perpendicular to the plane of the drawing. The pressurizing stay 111 is inserted in the holder 112.
An elastic pressure roller 115 serving as a pressurizing member is rotatably supported by bearings at both ends of a metal core. Above the pressure roller 115, an assembly of the heater 113, the holder 112, the film 114 and the stay 111 is positioned, with the heater 113 facing downwards, parallel to the pressure roller 115, and longitudinal ends of the pressurizing stay 111 are pressed downwards with urging members (not shown) to press the lower face of the heater 113 to urge it downward, through the film 114, to the upper surface of the pressure roller 115 against the urging means force of an elastic layer thereof, thereby forming a pressurized nip portion N of a predetermined width.
The pressure roller 115 is rotated clockwise as indicated by an arrow and with a predetermined peripheral speed by unillustrated driving means. By a pressurized frictional force at the pressurized nip portion N at the pressure roller 115 and the film 114 in the rotation of the pressure roller 115, a rotating force is exerted on the cylindrical film 114, which is thus driven counterclockwise as indicated by an arrow outside the holder 112, in sliding contact with the downward face of the heater 113.
In a state where the pressure roller 115 is rotated to also rotate the cylindrical film 114 and the heater 113 is energized, showing a rapid temperature increase and controlled at a predetermined temperature, a recording medium S bearing an unfixed toner image t is introduced between the film 114 and the pressure roller 115 at the pressurized nip portion N, in which the recording medium S, with a toner image bearing surface thereof in close contact with the external surface of the film 114, is pinched and conveyed together with the film 114. In the course of such conveying process, the recording medium S is heated by the heat of the film 114, which is heated by the heater 113, whereby the unfixed toner image t on the recording medium S is heat fixed thereto by heat and pressure. After passing the pressurized portion N, the recording medium S is separated by a curvature from the film 114 and is conveyed for discharge.
The image heating apparatus of the aforementioned film heating type, capable of employing a heater of a low heat capacity as the heating member, can achieve an electric power saving and a shorter wait time in comparison with the prior apparatus of a heat roller type or a belt heating type.
In such image heating apparatus of on-demand type, the heater 113 and the pressure roller 115 are mutually pressed by pressurizing both longitudinal ends of the pressurizing stay 111 and the pressure roller 115 with urging members such as spring. In such pressurizing configuration, even a slight bending in the pressure roller 115 or the pressurizing stay 111 tends to result in a situation where a pressure at a longitudinal center of the nip is smaller than a pressure at longitudinal ends of the nip. Such uneven pressure distribution renders the nip width, in the conveying direction of the recording medium, uneven over the longitudinal direction, thus often resulting in an uneven image fixing property. In order to compensate such unevenness in the nip width distribution, a heater holding surface of the holder 112 is made somewhat thicker in a longitudinal central portion than in both end portions, in such a shape that the heater 113 is bent and positioned closer to the pressure roller 115 in the longitudinal central portion than in both end portions (such shape being hereinafter called a crown shape).
Also in order to discharge the recording medium without creases, it is already known to form the pressure roller in an inversely crowned shape, namely a shape where the diameter is larger in both longitudinal end portions than in a central portion. In the pressure roller of such inversely crowned shape, the pressure roller has a peripheral speed larger in both ends portions than in the central portion, whereby the recording medium is subjected to a tensile force from the center to both ends in the conveying process through the pressurized nip portion. Such phenomenon is considered to suppress generation of creases.
However, a mechanism of suppressing crease generation on the recording medium does not necessarily depend only on the peripheral speed difference between the central portion and the end portions of the pressure roller. The aforementioned bending (crowning) of the heater 113 for compensating the unevenness in the nip with of the pressurized nip portion, if made excessively large, may cause creases in the discharged recording medium even if the pressure roller has an inversely crowned shape. An increasing crowning in the heater corresponds to an increase in the nip width (width in the conveying direction of the recording medium) at the longitudinal central portion of the nip. Thus the mechanism of suppressing crease generation on the recording medium is considered to depend not only on the peripheral speed difference between the central portion and the end portions of the pressure roller but also to be delicately related with the difference of the nip width between the longitudinal central portion and the end portions of the nip. In any case, an excessively large crowning of the heater is disadvantageous for crease formation in the recording medium.
On the other hand, in a portion immediately after being discharged from the pressurized nip portion N of the film 114 and the pressure roller 115, the recording medium S is released from a constriction by the pressurized nip portion N and shows a thermal dilatation as shown in FIGS. 8 and 9, thus generating undulations Sa in the conveying direction. In case such undulations are generated, a convex portion of such undulations contact the film 114 for a longer time, whereby a convex portion of the undulations Sa in the recording medium S tends to receive an excessive heat in comparison with a concave portion. Such undulations Sa are conspicuous in a resinous film such as an OHP sheet or a glossy film.
Particularly in case the film 114 is formed by a sleeve constituted of an elastic layer, a releasing layer and a metal film and having a certain heat capacity (for example a heat capacity per unit area is 0.1 J/cm2·K), a convex portion in the undulations Sa generated in the recording medium S receives an excessive heat in comparison with a concave portion. Since such excessive heat deteriorate the surface smoothness in the convex portion, there will result, as shown in FIG. 9, a deteriorated transparency along the convex portion of the undulations Sa in case the recording medium S is an OHP sheet, or an unevenness in the luster in case the recording medium S is a glossy film. Such image unevenness seems to be appeared in the form of flames, hereinafter it is referred to as a fire mark.
Such fire mark tends to become more conspicuous in case the pressurized nip portion N, formed by the heater 113 and the pressure roller 115 across the film 114, has a small crown amount C in the longitudinal direction of the nip (for example C=100 μm for a nip length L=220 mm), and become less conspicuous as the crown amount C is larger (for example C=300 μm for a nip length L=220 mm). This is presumably because, as explained before, a small crown amount C of the pressurized nip portion N increases an ability of spreading the recording medium S during conveying in the pressurized nip portion N, thereby giving a larger stress to the recording medium S and enhancing the undulations Sa. However, in case of selecting a large crown amount C for giving an emphasis on the influence thereof on the fire mark, the ability of spreading the recording medium S during conveying in the pressurized nip portion N becomes lower whereby the creases become enhanced in a recording medium S of low stiffness such as a thin paper.